1. FIELD OF THE INVENTION
The present invention relates to a fin-tube heat exchanger which can be used for a heat pump type air conditioner using air as a heat source and, particularly, which can also be used outdoors.
2. DESCRIPTION OF RELATED ART
A conventional fin-tube heat exchanger is described below with reference to FIGS. 1, 2 and 3.
As shown in FIG. 1, a fin-tube heat exchanger generally comprises a group of fins 2 (2A, 2B) which are arranged in parallel at predetermined intervals so that an air current 1 flows between the respective fins 2, cylindrical fin collars 3 (3A, 3B) which are formed in the fin group 2 with a predetermined longitudinal pitch and lateral pitch, and a group of tubes 4 which are respectively inserted into the fin collars 3 and then joined to the fin collars 3.
The fin 2A of a first example of conventional fin-tube heat exchangers shown in FIGS. 2 and 3 has circular seat portions 6 respectively provided concentrically with the fin collars 3A on the surface of the fin 2A, and a plurality of ridge portions 7 which are formed in each line of the fin collars 3A outside the fin collars 3A of the seat portion 6, and each of which has a ridge linearly extending in the longitudinal direction of the fin 2A. When the air current 1 flows between the respective fins 2A, heat transfer can thus be promoted due to the turbulence promoting effect.
The fin 2B of a second example of conventional fin-tube heat exchangers shown in FIGS. 4 and 5 has a plurality of raised portions 8 which are provided between the adjacent two fin collars 3B on the surface of the fin 2B in the longitudinal direction thereof, whereby heat transfer can be promoted by the boundary layer leading-edge effect when the air current 1 flows between the respective fins 2B.
However, when a heat exchanger which uses the fin 2B shown in FIGS. 4 and 5 is used outdoors for a heat-pump air conditioner and is operated for heating, if the outside air temperature is decreased, frost adheres to the leading edges of the raised portions having a high efficiency of heat transfer, and the leading edges of the raised portions are closed after a while. The heat exchanger thus has the problem that the heat transfer efficiency is abruptly decreased, and the heat exchanger cannot be continuously operated over heating for a long time period.
When a heat exchanger having the fins 2A shown in FIGS. 2 and 3 is used outdoors for a heat pump-type air conditioner and operated for heating, since the fins 2A have no raised portion, even if the outside air temperature is decreased, the exchanger can be continuously operated for heating over a longer time, than the heat exchanger having the fins 2B shown in FIGS. 4 and 5. However, the heat exchanger has the problems that (1) because heat transfer is promoted by the turbulence promoting effect, the heat transfer efficiency is not so high, as compared with the heat exchanger having the fins 2B shown in FIGS. 4 and 5 in which heat transfer is promoted by the boundary layer leading edge effect, and (2) it is impossible to significantly enhance the performance of a heat pump-type air conditioner and decrease the size thereof.